More specific debug line control.

[pserwylo]

Don't always draw all debug lines. Only specific types of lines at specific times. Allow certain debug info to be shown/hidden by:

• Shift + M: Movement direction lines.
• Shift + C: Collision bounds.
• Shift + G: Pathfinding grid.
• Shift + P: Pathfinding results.

If G + P are both enabled, then the parts of the pathfinding grid which were searched in order to find the resulting path are also shown.